Water quality sensor apparatus

Abstract: Water quality sensor apparatus comprising a substrate on which a plurality of sensors are supported in spaced apart relationship, each sensor being responsive to a different water characteristic and each sensor comprising a sensor region which, in use, is arranged to be contiguous with water the characteristics of which are to be sensed, which sensor regions are electrically connected to connector means via conductors supported on the substrate, the conductors and the connector means being arranged to be non-contiguous with the water in use of the apparatus.

Description: This invention relates to water quality sensor apparatus and more especially it relates to apparatus for sensing contemporaneously a plurality of different characteristics appertaining to the quality of water. It is known to provide, for pollution control purposes for example, water quality monitoring apparatus which comprises a submersible assembly of individually fabricated sensors, one for each characteristic to be monitored. European patent application 0493819A1 discloses a water quality checker comprising a sensor member body having a circular end portion provided with a plurality of kinds of water quality measuring sensors. These sensors hang down from a lower end surface of the sensor member body. In Patent Abstracts of Japan volume 13, number 395, Taisei Corp discloses apparatus to measure in real time different parameters of water, such as water temperature, chlorine content, dissolved oxygen, and pH. These known apparatus have several disadvantages including high cost of manufacture, comparatively large size and inconvenient accessibility to the sensors for maintenance purposes. It is an object of the present invention to provide an improved water quality sensor apparatus wherein one or more of the foregoing problems is substantially obviated. According to the present invention water quality sensor apparatus comprises a substrate on which a plurality of sensors are supported in spaced apart relationship, each sensor being responsive to a different water characteristic and each sensor comprising a sensor region which, in use, is arranged to be contiguous with water the characteristics of which are to be sensed, which sensor regions are electrically connected to connector means via conductors supported on the substrate, the conductors and the connector means being arranged to be non-contiguous with the water in use of the apparatus, characterised in that the substrate on which a plurality of sensors are supported is substantially flat. In alternative embodiments the substrate may be cylindrical and/or tubular, the sensors being carried on an inner or outer cylindrical surface. Sensors may be provided for dissolved oxygen, pH conductivity, and temperature. Reference electrodes may additionally be provided on the same substrate or alternatively they may comprise a separate assembly, and a redox sensor may also be provided. In one embodiment of the invention, a dielectric substrate is provided having supported on one side thereof reference electrodes, a dissolved oxygen sensor and a pH sensor, and on the other side thereof in juxtaposition therewith a temperature sensor, a redox sensor and a conductivity sensor. The sensors may be laid down on the substrate using thick film techniques. The reference electrodes may comprise a silver halide region to which on one side thereof an electrical connection is made from one of the conductors and having on the other side thereof an insulative dielectric filled with a soluble salt of the corresponding halide, to which in use, electrical connection is made by the water. The halide may be chloride and the salt may be potassium chloride. The pH sensor may comprise a layer of noble metal to which electrical connection with one of the conductors is made and over which is laid an oxide of antimony, ruthenium or iridium or other suitable metal to which in use electrical connection is made by the water. The noble metal may be platinum. The oxide may comprise a surface region of a metal which corresponds to the oxide or an oxide filler supported in a dielectric material. The conductivity sensor may comprise a plurality of conductors, between which the water in use is introduced. The temperature sensor may comprise a conductive stripe carried on the substrate, the temperature of the water being measured in dependence upon the resistance of the stripe. One embodiment of the invention will now be described by way of example only with reference to the accompanying drawings, in which; FIGURE 1 is a schematic block diagram of a water quality sensor system; FIGURE 2 is a generally schematic block diagram of a part of the system shown in Figure 1; FIGURE 3a is a plan view of one side of a sensor array; FIGURE 3b is a plan view of the other side of the sensor array shown in Figure 3a; FIGURE 3c is a sectional view on a line A as shown in Figure 3a of the sensor array shown in Figures 3a and 3b; FIGURE 3d is a sectional view on a line B as shown in Figure 3a of the sensor array shown in Figures 3a and 3b; FIGURE 3e is a sectional view on a line C as shown in Figure 3a of the sensor array shown in Figures 3a and 3b; FIGURES 4a and 4b are respectively a plan view and a side view of a thick film pH sensor; FIGURES 5a and 5b are respectively a plan view and a side view of a thick film reference electrode; FIGURE 6 is a circuit diagram suitable for four terminal conductivity measurement; FIGURES 7a and 7b are circuit diagrams appertaining respectively to clock low and clock high conditions of the circuit shown in Figure 6; FIGURES 8a and 8b are respectively circuit diagrams appertaining to reversed plurality modes appertaining to operation of the circuit shown in Figure 6; FIGURE 9 is a circuit diagram which is a modification of the circuit shown in Figure 6, comprising separated switching signals; and FIGURE 10 is a wave form diagram appertaining to the circuit shown in Figure 9. Referring now to Figure 1, a sensor system for water quality monitoring comprises a sensor array 1 which is immersed in water 2, the quality of which is to be monitored, the water being contained in a beaker 3. Signals from the sensor array are fed via electrical conductors 4 to signal conditioning electronics 5. Signals derived via the signal conditioning electronics are fed to a processor and display unit 6. Referring now to Figure 2, the sensor array 1 comprises a temperature sensor 7, a redox sensor electrode 8, conductivity sensor electrodes 9, a pH sensor electrode 10, a reference electrode 11 and dissolved oxygen sensor electrodes 12. Signals from the sensors 7 to 12 are fed to signal conditioning circuits 13 to 17 as shown in the drawing, which in turn are connected as shown in the drawing, to processor display and data logging units 18 to 22. Referring now to Figures 3a to 3d, the sensors 7 to 12 are all supported on a ceramic substrate 23, the pH sensors 10, the reference electrodes 11 and the dissolved oxygen sensor 12 being supported on one side of the substrate 23, as shown in Figure 3a, the temperature sensor 7, the redox sensor electrode 8 and the conductivity sensor 9 being supported on the other side of the substrate 23, as shown in Figure 3b. Sectional views through lines A, B and C are provided in Figures 3c, 3d and 3e, respectively to illustrate the pH sensor 10, the reference electrodes 11 and the dissolved oxygen sensor 12. Materials from which the sensors are fabricated are shown by means of a key 24 and the sensors are fabricated using the materials shown, using thick film techniques as will be apparent to those skilled in the art. The manner in which a pH sensor corresponding to the pH sensor 10, shown in Figure 2 and 3a, is constructed will now be considered in more detail, with reference to Figures 4a and 4b, wherein corresponding parts bear the same numerical designations. Referring now to Figures 4a and 4b, a pH sensor comprises a substrate of ceramic material 24, on which is supported a noble metal electrode 25, which in this case is platinum. An insulation layer 26 is provided which overlays the platinum layer 25 and from which is arranged to extend a solderable electrical contact edge 27, which is electrically connected to the platinum layer 25. A pH sensitive metal oxide region 28 is provided at one end of the platinum layer 25, the oxide comprising an oxide of antimony, ruthenium, or iridium or other suitable metal, which is contained in a printable medium such as a polymer dielectric paste. Referring now to Figure 5a and 5b, a reference electrode arrangement is shown wherein corresponding parts of the two figures bear the same numerical designations. The reference electrode shown corresponds to the reference electrodes 11, shown in Figures 2 and 3a, and comprises a substrate of ceramic material 29 which is arranged to carry a silver electrode 30, which provides a solderable interface region 31 to which electrical connection can be made. Overlaying the silver electrode 30, an insulation layer 32 is provided and at the end thereof remote the connection interface 31, a silver halide region 33 is formed which may, for example comprise silver chloride. Overlaying the silver halide region 33, a halide salt region 34, comprising for example potassium chloride, is provided in the form a printable medium, comprising a polymer paste. In the case of the pH sensor, as shown in Figures 4a and 4b the metal oxide region 28 is exposed to the water for pH sensing purposes and in the case of the reference electrode, as shown in Figures 5a and 5b, the halide salt region 34 is exposed to the water under test for sensing purposes. As shown in Figure 3b, conductivity measuring electrodes 35, 36, 37 and 38 are provided which facilitate a four terminal conductivity measurement. For the purpose of this measurement, outer electrodes 35 and 38 are used as source electrodes to pass a current through the water under test and inner electrodes 36 and 37 are used as sense electrodes responsive to a potential difference. As shown in Figure 3b, end portions only of the electrodes 35 to 38 are exposed to the water under test, the remainder of the electrodes being insulated by means of a passivation layer 39 which serves to protect them from the water. A circuit used for four terminal conductivity measurement is shown in Figures 6, wherein connections from electrodes 35 to 38 are made to corresponding input terminals 35a to 38a. The measurement method involves the use of electronic switches 40 to 43 which serve to convert a dc current from a source S into a ac square wave and a further set of switches 44 to 47 which serve to convert the square wave measured in the liquid back into a dc voltage which is applied to a differential amplifier 48. Capacitors 52 to 55 are provided for coupling purposes and serve an additional function as hereinafter described. The manner of operation of the circuit to produce the specified result will be readily apparent to those skilled in the art, and accordingly further description of the mode of operation is believed to be unnecessary. As shown in Figures 2, 7a and 7b however, the paths of the currents and potentials during each half cycle are illustrated by means of thick and thin broken lines respectively and it can be seen that the water under test experiences an ac signal with no dc component whilst the current source in the differential amplifier experiences a continuous dc current whereby low cost accurate dc electronics can be used to make an ac measurement. As shown in Figure 8a and Figure 8b, the relative phase of the clock pulse signals to the source and measurement electronic switches can be reversed. This has the effect of reversing the polarity of the signal as sensed at the differential amplifier, as illustrated in Figure 8a and Figure 8b. Thus by doing two measurements with the clock pulse signals alternately in-phase and out of phase and by taking the difference between the differential amplifier outputs for each measurement, effects produced due to offset voltages in the amplifier and measurement system, are effectively cancelled whereby they are eliminated so that high accuracy is afforded, which is especially important when measuring small potential differences. The circuit thus far described, lends itself to refinements which will now be considered. Firstly, switching signals to the source switches 40 to 43 are overlapped to give a 'make before break' action. This ensures that the current source or sources are not allowed to saturate during switching and additionally ensures that the capacitors 52 to 55 are allowed to discharge fully in case the current source and solution are isolated from each other. Secondly, the 'on' period of the switching signals to the measuring electrodes switches 44 to 47 are effectively shortened so that all switches are in an 'off' condition whilst the source switches are being switched. This ensures that any transient effects caused by the switching, or non-linear transient effects in the solution do not appear at the inputs to the differential amplifier 48. The capacitors serve in effect to hold the potential at the amplifier inputs during the 'off' periods. Referring now to Figure 9, the circuit shown corresponds substantially with that shown in Figure 6, but the clock connections have been re-drawn and numbered S1, S2, M1 and M2 which correspond with switching wave forms bearing the same designations as shown in Figure 10. The main advantages afforded by the four terminal conductivity measurement as just before described, stems from the use of switches to facilitate the provision of ac current from a dc source and the embodiment of circuitry which facilitates phase reversal to eliminate 'off set' effects. In order to facilitate redox measurement, electrode 8 is provided which may simply comprise an electrode of noble metal such as platinum. In order to facilitate temperature measurement, the temperature sensor 11 comprises a conductive stripe which is tapped to define a potentiometer, the conductive parts of the stripe being insulated from the water and from each other by the passivation layer 39. By providing in a system as shown generally in Figure 1, having a sensor array as hereinbefore described, the provision of apparatus is afforded which is inexpensive to fabricate, convenient to use and easy to maintain.

Claims: Water quality sensor apparatus (1) comprising a substrate (23) on which a plurality of sensors (7, 8, 9, 10, 11, 12) are supported in spaced apart relationship, each sensor being responsive to a different water characteristic and each sensor comprising a sensor region which, in use, is arranged to be contiguous with water the characteristics of which are to be sensed, which sensor regions are electrically connected to connector means (4) via conductors supported on the substrate, the conductors and the connector means being arranged to be non-contiguous with the water in use of the apparatus, characterised in that the substrate on which a plurality of sensors are supported is flat. Apparatus as claimed in claim 1, wherein the substrate is cylindrical and/or tubular, the sensors being carried on an inner or outer cylindrical surface. Apparatus as claimed in any preceding claim, wherein the sensors are provided for dissolved oxygen, pH, conductivity, and temperature. Apparatus as claimed in claim 3, wherein reference sensor electrodes are additionally provided on the substrate together with a redox sensor electrode. Apparatus as claimed in any preceding claim, wherein the substrate comprises a dielectric having supported on one side thereof reference sensor electrodes, a dissolved oxygen sensor and a pH sensor, and on the other side thereof in juxtaposition therewith a temperature sensor, a redox sensor and a conductivity sensor. Apparatus as claimed in any preceding claim, wherein the sensors are laid down on the substrate using thick film techniques. Apparatus as claimed in any of claims 4 to 6, wherein the reference electrode(s) comprises a silver halide region to which on one side thereof an electrical connection is made from one of the conductors and having on the other side thereof an insulative dielectric filled with a soluble salt of the corresponding halide, to which in use, electrical connection is made by the water. Apparatus as claimed in claim 7, wherein the halide is chloride and the salt is potassium chloride. Apparatus as claimed in any of claims 3 to 8, wherein the pH sensor comprises a layer of noble metal to which electrical connection with one of the conductors is made and over which is laid an oxide of antimony, ruthenium or iridium or other suitable metal to which in use electrical connection is made by the water. Apparatus as claimed in claim 9, wherein the noble metal is platinum. Apparatus as claimed in claims 9 or 10, wherein the oxide comprises a surface region of a metal which corresponds to the oxide filler supported in a dielectric material. Apparatus as claimed in claims 3 to 11, wherein the conductivity sensor comprises a plurality of conductors, between which the water in use is introduced and which serve to facilitate four terminal measurement. Apparatus as claimed in claim 12, wherein two source electrodes and two measurement electrodes are provided, the source electrodes being fed with ac current from a dc source via a switching inverter and the measurement electrodes being coupled to a differential amplifier via a switching circuit which serves to convert an ac signal measured to a dc signal which is applied across the differential amplifier. Apparatus as claimed in any of claims 3 to 13, wherein the temperature sensor comprises a conductive stripe carried on the substrate, the temperature of the water being measured in dependence upon the resistance of the stripe.

Inventors: ATKINSON JOHN KARL, BALL CLARE LOUISE, BIRCH BRIAN JEFFREY, CRANNY ANDREW WILLIAM JAMES, HEWINSON JOHN, MOSS ADRIAN JAMES, SIUDA PRZEMYSLAW RAFAL, WALTON PETER, ATKINSON, JOHN KARL, BALL, CLARE LOUISE, BIRCH, BRIAN JEFFREY, CRANNY, ANDREW WILLIAM JAMES, HEWINSON, JOHN, MOSS, ADRIAN JAMES, SIUDA, PRZEMYSLAW RAFAL, WALTON, PETER
Applicants: SIEMENS PLC
Application year: 1994
Main classification: G01N33
IPC codes: G01N27, G01N33